Method of forming monocrystals



1.. P. HUNTER mon oF FoRMmG MoNocRY'sms Dec. 10, 1957 Filed Deo. 18, 1953 F|G..1. A

. INVUVTOR.

LLOYD P. HUNTER AT OR NEY United States Patent 2,816,050 METHOD oF FoRMiNG MoNocRYsrArs Lloyd P. Hunter, Poughkeepsie, N. Y., assignor to International Business Machines Corporation, New York, N. Y., a corporation of New York Application December 18, 1953, Serial No. 399,092

Claims. (Cl. 14S-1.6)

This invention relates to forming of monocrystals, and especially to monocrystals of germanium and other semiconductive materials which are suitable for use in transistors and similar devices.

The peculiar electrical characteristics of transistors, which makes them suitable for use as amplifiers, oscillators, detectors and as non-linear electrical devices generally, occur with reliable uniformity only when the semiconductive body of the transistor is formed from a single crystal.

The common method for growing single crystals has been the Bridgeman method, which involves lowering a cone-shaped crucible containing molten material to be crystallized through a region in which a temperature gradient is maintained such that the upper portion of the region is above the melting point of the material, while the lower portion of the region is below that melting point. The Bridgeman method is not suitable for materials which expand on freezing, since the stresses developed in the material cause polycrystalline effects. For such materials, the common method of growing a single crystal, up to the present, has been the method discovered and described by Czochralski early in this century. Briefly, it consists of lowering a rod of pure germanium (or other material of which the crystal is being formed) into a crucible containing molten germanium. The crucible must be in a furnace having a controlled vertical temperature gradient such that the surface of the molten material in the crucible is at the melting point of germanium. The molten material below the surface is of course hotter than this melting point and the space above lthe surface is cooler. The rod of germanium is then very slowly retracted upward at a carefully controlled rate of speed, with the result that a single crystal of germanium is grown on the end of the rod. Great care must vbe exercised with that process, both with respect to temperature control and with respect to the motion of the rod, in order to prevent the development of a polycrystalline structure. The furnace temperature gradient must be continuously adjusted to cornpensate for Vthe change in location of the surface of the molten material as the crystal is withdrawn from it.

It is an object of the present invention to provide an improved and simplified method of growing monocrystals.

Another object is -to provide a method which will permit the use of simpler apparatus.

Another object is to provide improved and simplified apparatus for growing monocrystals.

The foregoing and other objects of the invention are attained by supporting a molten mass of the material to be crystallized so that it is laterally unconned, and cooling the molten mass by extracting heat from a limited locality of the mass so that the solidification of the mass proceeds from that locality. More specifically, a mass of germanium, for example, is placed in the bottom of a crucible, the mass being small enough so that when melted it does not touch any of the lateral walls of the crucible. A rod of suitable heat conductive material,

2,816,050 Patented Dec. l0, 1957 preferably having a pointed tip, is brought into contact with a portion of the molten pool, and then heat is slowly extracted from a point on the rod remote from the pool. The pool gradually freezes to form a single crystal, the freezing action proceeding radially from the point of contact of the rod as a center. The crucible and the rod should be of a material which will not wet the material from which the crystal is being formed.

Other objects and advantages of the invention will become apparent from a consideration of the following specification and claims, taken together with the accompanying drawing.

In the drawing:

Fig. 1 is a somewhat diagrammatic vertical sectional view, through a furnace adapted to carry out the method of the present invention; and

Fig. 2 is a horizontal sectional view taken on the line II-II of Fig. 1.

`Referring to the drawing, there is shown a heater o1 furnace 1 of generally cylindrical form in which is located a cylindrical crucible Z. On the bottom of the crucible 2 and spaced from the lateral walls thereof is a molten pool 3 of germanium or other material to be crystallized. Projecting downward into contact with the pool 3 is a rod 4 having its lower end pointed, and attached at its upper end by suitable means to a cover S for the furnace 1. Means is provided for evacuating this space within the cover 5 of the furnace 1. The evacuating means may be of any suitable construction and is illustrated diagrammatically as including a vacuum pump 6. The upper end of the rod 4 is provided with a cooling coil 7 of suitable construction. Although shown as an external coil, the cooling mechanism may take other forms, and may, for example, be within the rod 4.

In carrying out the method of forming a monocrystal according to the invention, a quantity of germanium is placed in the bottom of the crucible 2 and the crucible is then placed in the furnace 1. The cover 5 with the attached rod 4 and evacuating pump connections is then placed on the heater 1. The space within the heater 1 and the cover 5 is then evacuated to a suitable degree and the heater is then started to bring the germanium 3 to its melting point. Any suitable 'heating means and temperature control means may 4be used, since such means forms no part of the present invention. The entire interior of the furnace should be maintained substantially at the melting point of germanium.

The quantity of germanium placed in the crucible 2 should be such that when melted it will form a pool 3 which does not touch any of the lateral walls of the crucible. Heat is extracted from the pool through the rod 4 by cooling its upper end, as for example by means of the cooling coil 7. It is desirable to extract heat slowly, without any sudden drop in temperature at any locality within the furnace. As the heat is extracted, the germanium in the pool freezes. It freezes first adjacent the tip of the rod 4, and the freezing progresses outwardly.

Germanium expands on freezing by approximately 10% of its volume. In order that the crystal formed from the pool 3 may be a complete monocrystal, it is necessary that the edges of the pool never touch the walls of the crucible at any time during the cooling process. Consequently, the mass of material in the pool must be smaller than the mass required to cover the bottom of the crucible. The mass of germanium employed should be smaller than the bottom of the crucible not by the exact amount of space required for expansion of the germa nium on cooling, but an additional four or live percent of the area of the crucible s'hould be allowed for error.

If one edge of the pool touches one wall of the crucible during the cooling process, a polycrystalline formation will develop at that edge. The pool may form a -usable monocrystal Vthroughout most of its structure, but it will be polycrystalliney wherever it is laterally confined.

-In order that the liquidpool will -not spread -out "and touch-the -walls of -the crucible,the cruiblemust'lbe of a -material vwhich ldoes not wetthe material obwhich the erystalvis -beingfermed Therodalso should not Vbe wetv byt-t'he germanium. zItis also desi-rablethat the Crucible and-the rodv be of Vmaterial which-will not eontaminate the crystal. I nlthe case of germanium, `graphite orcarbon is preferredffor both the eruc-ible andtherod from bothfthese standpoints.

"Whilethe-present method is not necessarily limited-to use with-materials which expand-on freezing, -itdndsits greatest advantage with such materials.

The evacuation of the furnace during -the melting process isvvused only to prevent contaminationoftheY germaniumbyjthe air or other lgas -in-the furnace. Where contamination of thematerial by the gases inthe furnace isnotV a problem, evacuation-may-be omitted.

The bottom of the Crucible 2 on which the pool "rests may-be .eitherat or slightly concave. If lthe concavity is lsulicient to partially -coniine the. melt, polycrystalline effects are likely to develop.

The general temperature -within -the lfurnace vshould be maintainedat the meltingipoint of theV material during the freezingV process. -The slow extraction of heatA from one point inv-the germanium pool can then be readilyeifective to freeze'the entire pool.

yWhile I have shown and described aV preferred embodiment of my invention, other Imodifications thereof will readily'occurto those skilled in the artand I therefore intend my invention to'be limited only bythe appended claims.

I claim:

1. The method of forming a monocrystal comprising the steps of forming in a furnace a laterally unconned pool of molten crystallizable material, maintaining the atmosphere in said furnace substantially at the melting point of said material, bringing apointed member of heat conducting material into Contact with the pool, and thereafter solidifying said pool by slowly extracting heat from ya limited locality vof said pool by cooling a portion of said member spaced from said pool.

2. The method of forming a monocrystal as defined in claim 1, inrwhich said member is formed of a material which does not contaminate the crystallizable material.

3. The method of forming a monocrystal as defined in claim 1, in which the material is germanium.

4. The method of forming a monocrystal as defined in claim 1, in whichsaid furnace and lsaid member are of material which does not wet the material of which the crystal is being formed.

5. The method of forming a monocrystal as dened in claim 1, in whichsalid member is formed of graphite.

Proceedings of the Royal Society of London, article by Kapitza, Series A, vol. CXIX, pages 362, 363.

Buckley: CrystalV Growth, pub. 1941, pages 86-89. 

1. THE METHOD OF FORMING A MONOCRYSTAL COMPRISING THE STEPS OF FORMING IN A FURNACE A LATERALLY UNCONFINED POOL OF MOLTEN CRYSTALLIZABLE MATERIAL, MAINTAINING THE ATMOSPHERE IN SAID FURNACE SUBSTANTIALLY AT THE MELTING POINT OF SAID MATERIAL, BRINGING A POINTED MEMBER OF HEAT CONDUCTING MATERIAL INTO CONTACT WITH THE POOL, AND THEREAFTER SOLIDIFYING SAID POOL BY SLOWLY EXTRACTING HEAT FROM A LIMITED LOCALITY OF SAID POOL BY COOLING A PORTION OF SAID MEMBER SPACED FROM SAID POOL. 